Method of operation of a printing unit and printing unit for offset machine

ABSTRACT

There is described a printing unit ( 1 ) for use in an offset machine. The printing unit enables wider application of offset machines ( 1 ). This is achieved by the coating means and the water application means comprising a unit ( 28 ) consisting of a doctor blade ( 30 ) and at least one roller ( 29, 32 ) for transferring coating or water from the doctor blade ( 30 ) to the plate cylinder ( 15 )/blanket cylinder ( 14 ) of the printing unit. The unit ( 28 ) is arranged for pivoting so that water is transferred to plate cylinder, and coating is transferred directly to the blanket cylinder.

This application claims the benefit of Danish Application No. PA 199901408 filed Oct. 1, 1999 and PCT/DK00/00542 filed Sep. 29, 2000.

BACKGROUND OF THE INVENTION

The present invention concerns a method for operating a printing unit inwhich the printing unit comprises a doctor blade used for coating and asmoistening unit for applying water, and where the coating means and thewater application means are constituted by a unit comprising a doctorblade and at least one roller for transferring coating or water from thedoctor blade.

Offset machines are well-known within the art and are therefore onlydescribed briefly. A web or a sheet on which printing is to be performedis led around back-pressure rollers or transfer rollers. The web or thesheets are brought into contact with a blanket cylinder for beingapplied the print to the applied in each single printing unit in theoffset machine. The blanket cylinder is in contact with a plate cylindertransferring the colour print to be placed on the web. The platecylinder is in contact with a moistening unit and a inking unit applyingwater and ink, respectively. Thus, an offset plate on the plate cylinderis rotated whereby water susceptible parts are moistened by the rollersof the moistening unit. Then the ink susceptible parts of the offsetplate are supplied with ink from the ink rollers in the inking unit. Theprint image formed is then deposited on the blanket cylinder whichfurther prints the ink on the web or the sheet. Preferentially, it willbe a paper web but other materials may also be printed.

A printing unit according to the present invention may be used in atraditional offset machine, for example of the kind described inEuropean patent application no 767,058. The content of this patentapplication is hereby incorporated by reference as the printing unit maybe a part of an offset machine which is built up according to the sameprinciple and with the same paper delivering and paper receiving meansat the beginning and the finish of the printing unit as well ascorresponding means for transferring paper web or single sheets betweendifferent printing units disposed in succession can be used forimparting the web the finished print. Also, the same kinds of printingink may be used. Offset machines may be equipped with a coating unit.The coating unit will typically be constructed with a cylinder on whichthe coating is applied from a roller arrangement which is supplied froma vessel with clear coating.

In International patent application PCT/DK98/00303 there is described asystem of the type mentioned in the introduction which is improved andthereby enables a broader application and more efficient operation ofprinting units in offset machines, where the printing unit may be usedfor coating and water application. In this system, the coating isestablished indirectly via the plate cylinder. However, it is desirableto apply coating directly on the blanket cylinder due to quality andfinish in the formed print.

Furthermore, GB-A-2,119,711 discloses a stationary fountain comprising adoctor blade co-operating with a ductor. A lacquer roller isdisplaceable between a first position for transferring water via a platecylinder and a second position for transferring coating directly to theblanket cylinder. There is no disclosure that the doctor blade could bedisplaced together with the interactive roller as a unit.

SUMMARY OF THE INVENTION

It is the object of the present invention to indicate a method foroperating a printing unit and a printing unit for an offset machinewhich enables a wider use and a more efficient operation of printingunits in existing and new offset machines. Furthermore, it is an objectto indicate a moistening unit which simultaneously may be used forcoating and which also enables flexoprinting in an offset machine.

According to the present invention this is achieved by a method which ispeculiar in that the doctor blade and an interacting roller aredisplaced between a first position for transferring water via a platecylinder to a blanket cylinder and a second position for transferringcoating directly to the blanket cylinder.

The printing unit for use by the method is peculiar in that the coatingand water application unit is arranged slidable between a first positionfor bringing said at least one roller in contact with a roller engagingthe plate cylinder, and a second position for bringing said at least oneroller in direct contact with the blanket cylinder of the printing unit.

The method and the apparatus make it possible that the doctor bladetogether with the roller form a unit which is slidable between the twopositions.

By using such a method and such a unit it becomes possible to makeoffset machines so that they obtain wider application, andsimultaneously the process may run more efficiently as the coat is notapplied indirectly via the plate cylinder to the blanket cylinder. Theslidable unit may be designed so that it may be retrofitted on existingoffset machines.

Coating or water from the chamber is transferred to the blanket cylinderor the plate cylinder via a roller which preferably is a screen rollerin the form of an Anilox roller, and the liquid lying in the cups of thescreen roller is transferred to the blanket or plate cylinder. Transferof water to the plate cylinder occurs as a rubber roller is insertedbetween the screen roller and the printing plate of the plate cylinder.Transfer of coating occurs directly to the blanket cylinder from thescreen roller.

When the transfer unit is displaced to its second position for contactwith the blanket roller, it is also possible to run flexographicprinting. The blanket cylinder is provided with a printing plate, andthe plate cylinder is displaced out of contact with the blanketcylinder. Then flexo inks may be transferred from the chamber and thetransfer roller in the shape of an Anilox roller to the printing plate.

If a completely covering print is desired, a blanket may be used on theblanket cylinder as in the case of coating.

It will be possible to use separate doctor blades for inking/coating andwater application. However, it will also be possible to use one and thesame doctor blade for coating and water application.

In a coating unit, which typically is the last printing unit in anoffset machine, it is advantageous that the coating means only compriseone screen roller in the form of an Anilox roller for transferring thecoating which is applied directly form the doctor blade to the blanketcylinder.

Most machines will be provided with a frame with coupling means forsupporting a cleaning system consisting of a liquid spray nozzle andcleaning paper. In some cases the printing unit according to theinvention may be mounted in coupling means of this fame. Hereby, theneed for special adaptation of the machine frame is avoided. Hereby itbecomes particularly simple to modify an existing machine as thecoupling means located in the frame of the offset machine are re-used ascoupling means for the unit according to the invention.

The motor used for driving the screen roller is independent in order toadjust the rotational speed to different offset machines. Thus the unitdoes not need a special adaptation of the drive of the screen roller fordifferent offset machines. In the machine, there will only be need for asuspension which in its most simple form consists of four pegs or screwson a rack.

By using a unit according to the invention, which is based on a doctorblade, it will be possible to apply highly pigmented inks, as forexample metal enamels. This will not be possible with common offsetprinting units as pigments/inks will clog here and make impossible theformation of a quality print.

The unit according to the invention may also be use as a moisteningunit. In the known moistening units, an environmental problem arises. Inorder to transfer the moistening water with the present rollerarrangement, it is necessary to add solvents. At the moment, this hasbeen prohibited at several places.

Alternatively, it has been attempted to solve the problem by tefloncoating for forming a kind of mask with the purpose of avoiding inkdepositing in certain areas. This is known as dry offset and is adifferent process in principle. Thus teflon has been used forsubstituting the water application from the moistening rollers. Thissystem has an advantage as the paper is not moistened and thereby therisk that coating adheres badly do not arise.

Instead of using the traditional moistening units, there may be used asystem which comprises a doctor blade and a screen roller and a rubberroller between the doctor blade and the plate cylinder as described inthe above International patent application. This is advantageous asfaster operation than previously is feasible. The amount of water orwater sausage formed in a wedge-shaped interspace between the rubberroller and the plate cylinder may be varied by running with varied speedbetween the rubber roller and the plate cylinder. By running the rubberroller with greater speed it is thus possible to provide a greateramount of water in the wedge. The amount of water may also be adjustedby varying the slot width occurring between the rubber roller and theplate cylinder. The printing unit according to the invention is thusadvantageous in that the amount of water situated in the slot may bevaried according to need.

As printing unit may be intended for coating and as moistening unit, itwill be possible to use the same unit consisting of a doctor blade andtransfer roller for both water and coating.

By using a common moistening unit it will not be possible to applycoating. Due to the surface speeds, a great and unallowablecontamination of the surroundings will occur as coating will be sprayedfrom the periphery of the roller and from the ends of the rollers. Byusing the unit according to the invention for coating, it will bepossible to avoid contamination.

It is also possible that, together with a plate cylinder and a blanketcylinder, there may be provided two units according to the invention ofwhich one unit is used for coating and the other for water application.Hereby it becomes possible to provide stripes of coating and stripes ofink side by side on the plate cylinder. This is made possible as thedoctor blade may be divided up for giving off liquid/ink over a part oftheir length. Hereby is thus achieved the possibility of making printwith quite new effects.

BRIEF DESCRIPTION OF THE DRAWINGS

In the following, the invention will now be explained with reference tothe accompanying schematic drawing, where:

FIG. 1 shows a side view of a typical offset machine comprising fourprinting units,

FIG. 2 shows a partial view for illustrating a known printing unitcomprising a moistening unit and an inking unit,

FIG. 3 shows a view corresponding to FIG. 2 for illustrating anembodiment of a printing unit according to the above Internationalpatent application,

FIG. 4 shows a view corresponding to FIG. 3 for illustrating a firstembodiment of a printing unit according to the invention, and

FIG. 5 shows a view for illustrating a further embodiment of a printingunit according to the invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a traditional offset printing machine 1 comprising fourprinting units 2. The machine has a transport direction 3 for sheetsthat are printed. The sheets comes from a delivery station 4 and areconveyed to a receiving station 5 by means of a delivery arrangement 6comprising a conveyor belt 7. The conveyor belt 7 runs about two chainwheels 8,9. The single sheets are conveyed from the unit 4 via a path 10around an impression cylinder or back-pressure cylinder 12. The singlesheets are placed at a position indicated by 13. The sheets are thusplaced in an area between a blanket cylinder 14 and the impressioncylinder 12. The blanket cylinder 14 is in contact with a plate cylinder15. Besides the impression cylinder 12, the offset machine alsocomprises transfer cylinders 16 for the sheets.

The offset machine furthermore comprises gripping means for holdingsheets and a long row of rollers for moistening units and inking unitswhich are in connection with the plate cylinder. Since these arewell-known, they are not shown in FIG. 1 which serves as illustration ofthe structure of the offset unit. These rollers, however, appear in FIG.2.

FIG. 2 shows a printing unit 1 comprising an impression cylinder 12, ablanket cylinder 14 and a plate cylinder 15. These cylinders arerotating according to the arrows 17,18, 19. A moistening unit comprisesa container 21 for water. From the water container 21 the water is ledvia a system of rollers 22 to the last contact roller 23 which is incontact with the plate cylinder 15. The printing unit 1 furthermorecomprises an inking unit 24 comprising a number of rollers 25transferring ink from an ink container 26 to contact rollers 27 whichapply the ink on a soft printing plate (not shown) situated on the platecylinder 15. The printing plate located on the plate cylinder will thusbe imparted ink in the areas where water has not been applied from themoistening unit 20. The printing plate is usually an etched metal plate.

As a coating unit is built up in principle as the moistening unit 20,FIG. 2 may also be said to illustrate a coating unit. The coating willthus be conveyed up from the container 21 containing coating andtransferred via rollers 22 to the last contact roller 23 which is alsocalled the forming roller. However, a coating unit will preferably bemounted on the blanket cylinder 14 for avoiding undesired dirtying.

The embodiment shown have some environmental and printing disadvantages.Instead of using the existing moistening unit, the printing unit in FIG.2 may be modified as illustrated in FIG. 3.

In FIG. 3 the contact roller 23 is substituted by a unit 28 comprising adoctor blade system 30 and a screen roller 29, preferably an Aniloxroller of the kind also used for flexographic printing. The screenroller 29 may be mounted directly in the existing suspension. Betweenthe screen roller 29 and the plate cylinder 15 there is mounted a softroller 32, preferably a rubber roller. The unit 28 may, even by greatperipheral speeds, ensure a constant and uniform amount of water and/orcoating transferred to the plate cylinder 15. If the unit 28 is desiredto be used for coating, the rollers 27 of the inking unit are broughtout of contact with the plate cylinder 15. If the unit 28 is used forwater application, the inking unit 24 is kept in engagement with theplate cylinder 15.

The embodiment shown in FIG. 3 may be changed when it is only used forcoating. Thus the hard screen roller 29 may be used directly without asoft roller for coating. This will, however, necessitate the use of arubber blanket on the plate cylinder 15.

The printing unit shown will be very simple and easy to maintain. At thesame time, the system is easy to replace depending on whether theprinting unit is desired to be used for one or the other purpose. Thusit will be possible, according to wish, to use the existing moisteningunit concurrently with the unit 28 according to the invention.

When the unit 28 is used for water application, it will be easy toadjust the water amount in a simple way. Such an adjustment is difficultin traditional moistening units where the rollers are runningsynchronously with the plate cylinder 15. The rubber roller 32 may beprovided with its own motor which is driven independently of the platecylinder. This creates possibility of a differentiated periphery speedand thereby possibility of stemming up of greater or lesser amount ofwater in the wedge-shaped interspace 31 formed between the rubber roller32 and the plate cylinder 15.

In FIG. 4 is shown a first embodiment of a printing unit 1 according tothe invention. FIG. 4 differs from the printing unit shown in FIG. 3 bythe unit 28 being suspended pivotably about an axis 33 of pivot runningin parallel with axes of rotation 34 and 35 for the blanket cylinder 14and the plate cylinder 15. The unit 28 is shown in a first position 36,where the screen roller 29 is in contact with a soft roller 32 engagingthe plate cylinder 15, and a second position 37 where the screen roller29 is directly engaging the blanket cylinder 14. These two positions areused for water application (position 36) and coating (position 37),respectively.

FIG. 5 shows a further embodiment of a printing unit according to theinvention. In this printing unit there is simultaneous use of two units28. The unit 28 illustrated to the right in the Figure is used forapplying moistening water. The unit 28 shown to the left is used forapplying coating. Since it is possible to divide up the doctor bladeover its length, it will be possible to apply coating in stripes wherethe moistening unit does not apply any moisture. Such an effect will notbe possible in traditional printing units. The coating unit and themoistening unit as illustrated in FIG. 5 will function according to thesame principle as explained above with reference to the precedingFigures. Alternatively, the unit 28 in the left side may be used fortransferring coating and flexographic ink. If there is a blanket on theblanket cylinder 14, a completely covering ink will be printed, and if aprinting plate is placed on the blanket cylinder 14, a desiredflexographically printed image may be established.

1. A method for operating a printing unit in an offset machine in whichthe printing unit comprises a doctor blade system comprising a chamberforming a coating unit for coating and ae a moistening unit for applyingwater, wherein the doctor blade system and at least one transfer rollerinteracting with the doctor blade system for transferring coating orwater from the doctor blade system are displaced between a firstposition for transferring water via a plate cylinder to a blanketcylinder and a second position for transferring coating directly to theblanket cylinder wherein the coating and water application unitcomprises transfer rollers in the form of a screen roller and a rubberroller for transferring water from the doctor blade system to the platecylinder and one screen roller for transferring coating directly to theblanket cylinder.
 2. A method according to claim 1, wherein thedisplacement is a pivoting about an axis in parallel with the rotationalaxis of the plate cylinder and the blanket cylinder.
 3. A printing unitfor use in a method according to claim 1 in an offset machine,comprising means for coating and means for applying water, and where thecoating means and the water application means are constituted by acoating and water application unit comprising a doctor blade systemincluding a chamber and at least one transfer roller for transferringcoating or water from the doctor blade system, wherein the coating andwater application unit is arranged movably between a first position forbringing said at least one roller in contact with a roller engaging theplate cylinder, and a second position for bringing said at least oneroller in direct contact with the blanket cylinder of the printing unit,and wherein the coating and water application unit further comprisessaid at least one transfer roller in the shape of a screen rollertransferring coating directly from the doctor blade system to theblanket cylinder.
 4. A printing unit according to claim 3, wherein thecoating and water application unit comprising the doctor blade systemand the at least one roller is mounted in the offset machine in anexchangeable way with the existing moistening unit of the offsetmachine.
 5. A printing unit according to claim 3, wherein the coatingand water application unit is mounted pivotably in relation to the platecylinder and the blanket cylinder between one of the engagementpositions with the plate cylinder and the blanket cylinder.
 6. Aprinting unit according to claim 3, wherein the coating and waterapplication unit is provided with coupling means which are arranged forbeing connected releasably with coupling means in the frame of theoffset machine.
 7. A printing unit according to claim 3, wherein thetransfer roller is driven by its own motor.
 8. A printing unit accordingto claim 7, wherein said motor is a motor controlled by a line signalfrom the main machine.
 9. A printing unit according to claim 7, whereinsaid coupling means in the frame is coupling means for a cleaning unitknown per se for the plate cylinder.